1.8 Protection and security

Access Lists: with each file, indicate which users are allowed to perform which operations.

• In the most general form, each file has a list of pairs.
• It would be tedious to have a separate listing for every user, so they are usually grouped into classes. For example, in Unix there are threeclasses: self, group, anybody else (nine bits per file).
• Access lists are simple, and are used in almost all file systems.

Capabilities: with each user, indicate which files may be accessed, and in what ways.

• Store a list of pairs with each user. This is called a capability list.
• Typically, capability systems use a different naming arrangement, where the capabilities are the only names of objects. You cannoteven name objects not referred to in your capability list.
• In access-list systems, the default is usually for everyone to be able to access a file. In capability-based systems, the default is for no-oneto be able to access a file unless they have been given a capability. There isno way of even naming an object without a capability.
• Capabilities are usually used in systems that need to be very secure. However, capabilities can make it difficult to share information: nobodycan get access to your stuff unless you explicitly give it to them.

Are the following things access-based or capability-based protection schemes?

• Protection Keys
• Page tables

Access enforcement

Some part of the system must be responsible for enforcing access controls and protecting the authorization and identificationinformation.

• Obviously, this portion of the system must run unprotected. Thus it should be as small and simple as possible. Example: the portion of the systemthat sets up memory mapping tables.
• The portion of the system that provides and enforces protection is called the security kernel. Most systems, like Unix, do not have a securitykernel. As a consequence, the systems are not very secure.

• What is needed is a hierarchy of levels of protection, with each level getting the minimum privilege necessary to do its job. However, this islikely to be slow (crossing levels takes time).

File system security

The problem addressed by the security system is how are information and resources protected from people. Issues include the contentsof data files which are a privacy issue, and the use of resources, which is an accounting issue. Security must pervade the system, or the system is insecure,but the file system is a particularly good place to discuss security because its protection mechanisms are visible, and the things it protects are very concrete(for a computer system).

We're talking about some interesting stuff when we talk about security. For certain people who like puzzles, finding loopholes insecurity systems and understanding them to the point of breaking them is a challenge. I understand the lure of this. Remember, however, that everyone usingthese machines is a student like yourself who deserves the same respect that you do. Breaking into another person's files is like breaking into their home, andshould not be taken lightly either by those breaking in, or those who catch them. Uninvited intrusions should be dealt with harshly (for example, it's afelony to break into a machine that stores medical records). If you really want to play around with UNIX(R) security, get yourself a linux box and play to yourheart's content; don't break into someone's account here and start deleting files.